The present invention is with respect to pneumatic timers or time delay relays having a housing in which there is a bellows acted upon by a spring force for keeping the bellows in a stretched out or pushed together condition, and a bellows driving unit by which, at the start of a timing operation, the bellows is stretched out or pushed together while taking in or letting off air through a valve opened by the driving unit, and then the bellows is moved back into its starting position by the spring force at a rate fixed by an adjustable choke joining up the space within the bellows with the outside atmosphere. When the bellows is back in its starting position, a sensing unit is worked which gives a signal.
A delay relay or timer on these lines is to be seen in German Offenlegungsschrift specification 2,204,667. Its adjustable choke is made up of a turning plate with a choke groove in one face thereof, the cross-section of the groove changing smoothly in a direction round the plate, which has its grooved face placed against a fixed plate having a hole therein running into the groove. By using such a choke with a grooved plate, time adjustment of the timer may be undertaken within a wide range. The groove in the plate may be designed to give a linear or a logarithmic change in time with a change in the angle of the grooved plate.